To all the people saying re-building the Abingdon branch would be a waste of money and instead money should be spent on buses even though the ones running at the moment are getting stuck in congestion I would ask the following question, which of the following statements is most likely to be true:
1 - All these motorists are desperate to use the bus but cant get a seat so they have to use their cars and look jealously on the people using the bus wishing they could be on it.
2 - The buses have already got all the people that want/need to use the bus and the only way you are going to get motorists out of their cars and cut congestion is to provide another means of transport. i.e. Trains
With some bus services they are called things like the "Hopper" or the "Nipper" because instead of being out and out profit making services they are more designed to link people with local facilities or places where they can then travel on to further destinations so what I would do is take that concept and adapt it for rail by doing the following: Taking Autotank's post #36 suggestion I would run a train starting at Wallingford from start of service till about 10am and then from 5pm till end of service to Cholsey I would then reverse to call at Didcot from there the train would carry on to Grove.
Looking at Google maps like pretty much everywhere there has been mass house building both in Grove and Wantage and no doubt there will be more and the problem with building a station on the mainline is you are encouraging people to get in to their cars to drive to the station and the problem is once they are in there car they may change there mind about using the train and do the whole journey by car so what you need to do is bring the station as close to them as you can so it is easy for them to walk/cycle too it so while I would leave space for a station to be built on the mainline in the future I would build a branch running south either to the east or west of Grove but most likely the east running beside the A338 Station Road towards Wantage with one or two simple halt style stations, at the king Alfred way roundabout I would tunnel under it and the A338 pop up the opposite side then run the line between the trees/greenery and the A338 to terminate next door to the Sainsburys petrol station here:
https://maps.app.goo.gl/vCLjWE3Sm61PQaRM8. I would then reverse and go back to Didcot reverse again and head north.
A few people have pointed out that the Abingdon junction is in the wrong direction so I would change it to a triangle junction then run to a new station (single platform/bus shelter should do) built on the site of the old one in the Waitrose car park and get Waitrose to stump up some cash to help pay for it as in the mornings they would get commuters coming in to buy food for their lunchtimes or food for their journey if they are going further afield and in the evenings people doing there shopping after getting off the train before going home or picking up a microwave meal/takeaway if they cant be bothered to cook.
From there reverse again then carry on to Oxford, from Oxford carry on north on to the rebuilt Witney branch. First stop would be Eynsham and there are a couple of empty fields between the already built houses and the old railway line which a developer would love to build on and considering any new houses are likely to be brought by people using the railway to commute a large donation from the developer to get the line built should not be a problem, from there you then carry on to Witney.
Sadly the line in and out of Witney and the station site have been completely obliterated and unless you were willing to spend possibly millions on demolition to rebuild the railway on its original alignment then that part of the line has gone forever therefore I would try and thread it through and put a station as close as I could to the ducklington/A40 roundabout here
https://maps.app.goo.gl/cotyVRhFFAfipUSt8 there is already a cycle path to Witney so you just add as many local buses running to and from the town center as you can and any coming off and on to the A40.
From there you have got a clear run to Carterton and Brize Norton. At Carterton and Brize not only do you have massive housing development but you also have the added bonus of the RAF personal who live on the base but also the ones who commute in. Looking at the geography of the area you have the railway in the south, the airfield in the middle and Carterton and Brize with what looks like expanding development in the north so I might look at building a new alignment to the north of the current development so if they keep building north then the station ends up in the middle of the development so makes it more convenient for more people.
I would also look at extending to Lechlade because while it is not huge it is a tourist honeypot so linking it with Oxford with a convenient train should be a decent money maker.
Again reverse back up to Oxford then a run down the Cowley branch then back to Oxford then Didcot - Cholsey and back to Wallingford I would also run a service in the opposite direction.
During the day so the Cholsey a Wallingford railway can keep operating I would either terminate at Cholsey or just miss both of them out altogether.
This plan would have the following advantages:
Get people out of there cars so cutting congestion
Give more connections which would give more options for both people looking for work and companys looking for employees.
Providing direct links between places, for example while I dont know what the buses are like in the area I very much doubt you can get a direct bus say from Wallington to Wantage or Wantage to Brize Norton without at least changing once and if you were trying to drive in the mornings or evenings it would take ages.
Brings back on to the network a lot of places that had a railway then lost it.
I dont know what house prices are like in Oxford but I am guessing they are extortionate as not only do you have locals trying to buy but you also have people moving from other areas to work and land lords looking to convert houses in to student accommodation so if the Witney line was re-opened workers and students could commute in thus easing house prices in Oxford.